DSL problem

Trying to troubleshoot what I thought was a basic problem. A DSL
modem installed in a construction trailer not working. There is a 6
pair outdoor phone cable feeding the trailer from the main building. 5
phones work perfectly fine in the trailer. For some reason I can't get
the DSL "happy" light to light up on the modem in the trailer. I
tested the modem inside the main building just before it leaves the
building and it worked fine. I swapped pairs on the outdoor cable, but
no help.
I know distance is not an issue because I tested that too. I "looped"
back the DSL line back down to where it comes in main telephone room
( basically doubling the distance) and it works fine. So I'm thinking
this is more of an inteference issue. Any ideas where to start? The
trailer is next to an MRI facility, not sure if that matters.

This is a single pair modem. As I said it works right before it leaves
the building. I tried all 6 pairs going to the trailer ( 5 phones
work out there now, and I confirmed all pairs are good).
I googled this topic and they say it might be something electrical
interfering. One site suggested shutting off electrical breakers
except the outlet powering the modem and go from there. It could be a
reversed polarity outlet. I'm really stumped. The trailer is only 100
feet away from the main building.

Uhh, it is CAT5 or better wire, right?
Replace one at the 50' point & see if it'll work there. I'm betting on
something wrong with the wires. REgular phone cable will never work
over 100'.
You said the modem was in the building and you're running 6 pairs 100'?
Why? Better to have the modem in the trailer. Assuming you really mean
"modem". What is your "modem"? Why do you think 100' isn't too far
away? What kind if wire is used to/from the modem in/out? Are you using
the shielded type wires? Are you ignoring the ground pins or using
them? Etc..

Very good question. He should have told us without your asking.
I once knew what each light meant. I've forgotten but other people
know.
Mike, you also didn't answer the question Hallerb asked first. Can
you connect a phone to that pair and get a dial tone. Can you do this
in the main office and not in the trailer? Or do you have DSL
without dialtone?.

Update: Well I tried turning off all breakers in the trailer, I also
tried reversing the plug. No help.
I asked one of the MRI physicists if anything might intefere with the
DSL in the 1400 KHz range. He said no.
I still had an old DSL modem that was previously thought to be
defective by the contractor. I plugged it in and it worked. Now
everything is fine.
The only explanation is that these 2 modems are made by different
manufacturers and there are probably different tolerance levels
between the 2 involving interference.
Thanks for everyones help.

Like I said, I did a loopback test of over 900 feet of regular 100
pair Cat 3 riser cable: From where it comes into the main building in
Ma Bells' frame closet, 450 feet away to the communicaton closet right
before it leaves the building to go out to the trailer, then looped
back 450 feet to Ma Bells frame again. It works. Distance is not the
issue.

The outdoor cable is called "Alpath". It is basically a rigid 6 pair
outdoor cable in thick jacket filled with silicone.
The modem I am trying to get working is in the trailer. It looks
basically like a broadband modem with wireless capability.
What I said was I tried out the modem box in the main building and it
works.

Sounds like you have the situaiton in hand so all should be well now.
Just for the heck of it though a single loopback test isn't proof that
it's not distance related if it's the wrong type wiring. It only means
that one pair by itself works OK. And CAT3 cable explains a lot,
actually.
IMO if you have more problems later, which is likely what happened
that they originally said that modem was defunct, you need to look into
all the suggestions that have been made to you. When it quits working
again and if you come back here, try to include the missing details as
well as anything else relevant you can think of. Many people had good
suggestions and questions which you seemed to ignore. You have wiring
problems.
Best of luck,

If there is any "bridge tap" ahead of the modem, that may be the cause
of the "no train" at the far end.
Dedicate a pair all the way from the demarc/SNID to the back of the
modem. Make sure there are no "extensions" tapped onto the pair - make
it a "dedicated" pair.
Until DSL was introduced, bridge tapped plant was common in the telco
field. The "same" pair would extend off of a main cable and run down
more than one county road. This "bridge tap" would prevent DSL from
working. Dedicating the pair (removing all the taps) would usually
condition the pair to work well with DSL - if within range of the DSLAM.
Bridge tap KILLS DSL.
Also, Cat5 or 6 or whatever is NOT required to provide a good signal to
a DSL modem. Generally, if you can TALK on it, DSL will work if the
modem is within range of the DSLAM. Cat5 won't hurt, it is just
overkill in most POTS installations - and that is what DSL is delivered
over: POTS (Plain Old Telephone Service) lines.
DSL is delivered all over the world on Cat 2-1/2 (!) cable that has been
in service since the '60s. There weren't even data "categories" then.
Cat5 cable is only needed for the network BEYOND the modem if the
ethernet is extended more than a few feet from the modem.

Oh, but is DOES come over a "phone line". It just has no dial tone.
That is called "Stand Alone DSL".
It is delivered over the exact, same pair that runs to that station -
with NO dial tone on it.
Troubleshooting "dry" DSL is a royal PITA. I often would "layer" C.O.
dial tone (talk line) on the pair during repair. Otherwise, waiting for
the DSL tester to train-up (sync) at each test point in the circuit was
time consuming and frustrating. With dial tone, it's MUCH easier: No
dial tone? The pair is open or seriously faulted somewhere.

Update: now the old modem, the one that worked for a while, stopped
working.
Then I remebered that theAlpath cable goes through a machine room
with these huge commercial lights ( like the kind you see lighting
Home Depot, Lowes that take a while to turn on). I shut off the
lights and both modems work now. So the fact that the phone cable
running close to these lights had something to do with it.

Can you hang a stick of EMT in the area of the lights, and run the cable
through that? Or maybe reroute the cable? Looks like your original hunch
was right, it was just the wrong suspect causing the interference. If
the lights are on the same mains power transformer can as the modems,
the interference could be coming through on the power side, as well. I
get static on all my am radios that use wall power when neighbor turns
on his damn security light.
aem sends....

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